This invention relates to a process for the production of phosphorus from phosphate, and particularly low-grade phosphate, rock. As used throughout this specification, the term "low-grade phosphate rock" is defined to mean phosphate rock containing a total amount of at least 5% by weight of iron and at least 5% of aluminum calculated as ferric and aluminum oxides respectively, and the term "high-grade phosphate rock" to mean phosphate rock containing a total amount of less than 5% by weight of each of iron and aluminum calculated as ferric and aluminum oxides.
There are several known processes for the production of elemental phosphorus from phosphate rock and similar ores. One such process is the blast furnace technique, wherein coke is heated in a blast furnace with phosphate rock and silica to produce a molten slag and hot reaction gases. In a further known process, phosphate rock in a pulverized state is introduced to a fluidized bed in combination with a carbonaceous material such as, for example, finely divided coke. In each case, elemental phosphorous is subsequently recovered from the hot reaction gases by the use of known condensation techniques.
Phosphorus produced by either of these processes can be utilized in various ways. It is however, a problem of the superphosphate industry that the present invention is, in particular, directed towards overcoming. Reference is now made to this problem.
One process of producing triple superphosphate involves the oxidation of elemental phosphorus to the pentoxide form, phosphoric acid in a pure state subsequently being obtained therefrom. Triple superphosphate is then formed by reaction of this phosphoric acid with calcium phosphate-rich high-grade phosphate rock. The superphosphate so produced is of good quality but the cost of production is high, due, at least partly, to the cost of high-grade rock.
In the other main process for triple superphosphate production, the so-called "wet process", calcium phosphate in high-grade phosphate rock is reacted directly with sulphuric acid to produce calcium sulphate and phosphoric acid. The phosphoric acid is then used in the production of triple superphosphate by further reaction with high-grade phosphate rock.
It has been considered for more than 150 years that low-grade phosphate rock is unsuitable for phosphoric acid production by the wet process, due to the concurrent production of undesirable amounts of aluminum and ferric hydrogen-phosphates. In low-grade phosphate rock, the higher content of ferric and aluminum oxides leads to the production of a very viscous phosphoric acid. If this acid is subsequently used to produce triple superphosphate, the product is "sticky" and of poor quality.
The discarding, as useless, of low-grade phosphate rock for the foregoing reasons is uneconomic, and wasteful of natural resources, as the phosphorus pentoxide content of such rock is frequently very high.
The use of low-grade rock in reduction processes for the production of phosphoric acid, however, reveals further problems. Most importantly, the use of low-grade rock in the hitherto known reduction processes causes technical problems due principally to the ratio of acidic to alkaline oxides in the molten reaction products.
The use of low-grade rock in reduction processes also results in the production of excessive amounts of ferro-phosphorus. Whilst some ferro-phosphorus is used commercially for example, in the iron foundry industry for the production of cast iron for fine iron lace work -- the vastly increased quantities that would be produced by the widespread use of low-grade phosphate rock would not be commercially useable. Hence, a good deal of the phosphorus and iron contained in these rocks as mined would be useless. This would not only be uneconomical but would additionally present a disposal problem.
The principal object of the present invention is to provide a process for the production, from low-grade phosphate rock, of elemental phosphorus which can subsequently be used for many purposes, including those of phosphoric acid and triple superphosphate production, and which does not suffer from the above-described disadvantages. Due to the relative cheapness of mined low-grade rock per ton as compared with high-grade rock, such a process consequently allows for the production of elemental phosphorus and, eventually, tirple superphosphate far more cheaply than heretofore possible.